Motor and its permanent magnet

ABSTRACT

The challenge to be solved by the present invention is the miniaturization of a 1-300 W class of motor. This can be achieved by using a hollow-cylinder shaped anisotropic bonded magnet magnetized in a 4-pole configuration. The anisotropic bonded magnet has a maximum energy product approximately 4 times greater than the conventional sintered ferrite magnets. The use of a 4-pole configuration shortens the magnetic path length of the individual magnetic circuits and the magnetic force contributing to the torque is increased. When the torque is kept the same as in the conventional motor, the length of the electromagnetic rotor core and the axial magnet length can be reduced. In this fashion, 1-300 W class motors can be reduced in size.

[0001] This is a patent application based on Japanese patentapplications No. 2002-276194 and No. 2001-375159which were filed on Sep.20, 2002 and Dec. 10, 2001, respectively, and which are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention is related to a DC brush motor and apermanent magnet used within. In particular, the present invention isrelated to a DC brush motor and associated permanent magnet whose smallsize and high torque are made possible by the use of an anisotropic rareearth bonded magnet as the permanent magnet. The present invention isvery effective for example in 1-300 W high-performance small-size DCbrush motor applications.

[0004] 2. Background Art

[0005] [Patent Document 1]

[0006] Published Unexamined Patent Application Number 2001-7691A

[0007] [Patent Document 2]

[0008] U.S. Pat. No. 2,816,668

[0009] [Patent Document 3]

[0010] U.S. Pat. No. 3,060,104

[0011] Prior to 1960, small-sized motors did not use magnets, but wereinduction brush motors. From 1960, with the appearance of affordableferrite magnets with a maximum energy product (BHmax) on the order of 4MGOe, small-sized brushless motors with a power consumption on the orderof 1-300 W appeared, and have been used for the last 40 years. Theconfiguration of these motors comprises 2-pole or 4-pole sinteredferrite magnets tiled on the inside perimeter of the housing, in thecenter of which is an electromagnetic rotor core wound with coils. Whenthe motor is driven, the direction of the current flowing through thecoils is changed via the brushes arranged on the rotational axis, andthe Lorentz forces, which arise from the interaction between the currentand the magnet field derived from the peripheral sintered ferritemagnets, induce rotation of the electromagnetic rotor core.

[0012] In recent years, there has been a demand for the miniaturizationof such small-sized motors, however this has not been realized becausesintered ferrite magnets with thin enough wall thickness cannot bemanufactured due to the shrinkage of sintered ferrite magnets duringsintering. Moreover, high-output motors could not be realized assintered ferrite magnets have a low attractive force.

[0013] In addition, if one attempts to make a large-size motor in orderto achieve high output, there is no alternative but to make a 4-polemotor, as the arc length is too great for a 2-pole motor. In this caseof a 4-pole motor using sintered ferrite magnets, the size and weightare increased, and it is not possible to improve the motor performanceindex (torque constant/volume). Furthermore, as the shape of sinteredferrite magnets differs depending on the environmental conditions, suchas humidity and the sintering conditions, it is difficult to achievetiled sintered ferrite magnets of exactly the same dimensions. Inaddition, it is necessary to individually arrange said sintered ferritemagnets in said motor housing. Because of this, the problem of squeakingand rattling can occur due to uneven torque resulting from errors insymmetry of the magnetic field made during precision arrangement. In thelatter part of the 1990's, an anisotropic bonded magnet with superiormolding properties, and superior magnetic properties of a maximum energyproduct (BHmax) no less than 14 MGOe, or four times that of a ferritemagnet, appeared on the scene and investigation into its application tomotors began.

[0014] However, these magnets were not adopted because when motormanufacturers simply tried to replace the ferrite magnets ofconventional small-sized brush motors with these magnets having fourtimes the maximum energy product, the motor properties only increased onthe order of 20%, and because the back yoke needed to be doubled, thesize actually increased. In addition, as the motor properties depend onseveral factors such as armature shape and properties, back yokethickness and material, coils, etc., the increase in properties couldonly be expected to be on the order of 20% and therefore these magnetshave not been adopted in recent years.

SUMMARY OF THE INVENTION

[0015] The purpose of the present invention is to solve theaforementioned problems that have long plagued the small-sized brushmotor industry by either reducing the volume of the motor by ½ whilemaintaining the same properties of the conventional motor, thus greatlyreducing size and weight, or by improving the motor properties twofoldwhile reducing the volume by 20% compared to the conventional motor,thus greatly improving the properties.

[0016] In other words, a motor with high properties, such as twice theperformance index T of the technological benchmark of the conventionalmotor using sintered ferrite, can be offered.

[0017] At the same time, by minimization of uneven torque, the quietnesscan be improved, and the process of gluing several magnets can beomitted from the manufacturing process.

[0018] Furthermore what is especially favorable is that compared tosintered ferrite magnets, less than ¼ the amount of magnet need be used,thus drastically cutting down on the necessary resources while at thesame time offering a high-performance motor.

[0019] The anisotropic rare earth bonded magnet of the DC brush motor asdescribed in the first aspect, which comprises a permanent magnetarranged on the inside wall of its housing and an electromagnetic rotorcore arranged in the center, is characteristically a hollow cylindermagnetized with at least 4 poles. The following mechanism, operation andadvantages of the present invention, which are generally becomingpopularized, will be introduced in comparison to the 2-pole (ferrite)motor.

[0020] The anisotropic bonded magnet adopted in the present invention isa magnet such as those formed by the production methods in PublishedUnexamined Patent Application Number 2001-7691A, U.S. Pat. No. 2,816,668and U.S. Pat. No. 3,060,104 as set forth by the applicants of thepresent patent, for example those magnets that are strongly magnetizedalong one axis and are manufactured by resin molding of NdFeB-basedmagnet powder. These magnets have a maximum energy product (BHmax) noless than four times that of the conventional sintered ferrite magnets.After a very devoted investigation by the inventers of the presentinvention into how the potential of these anisotropic bonded magnetscould be harnessed, they found that there would be great advantages inusing these magnets especially in 1-300 W small-sized brush motors. Byusing this anisotropic bonded magnet with high properties, the magnetthickness can be reduced, and at the same time the length of themagnetic path of the magnetic circuit of each magnetic pole can begreatly reduced by making four or more magnetic poles. Because of this,what was once thought to be impossible, has now been realized withepoch-making results of reducing the motor volume by ½ compared to theconventional motor while keeping the torque properties the same, thusresulting in a small, light-weight motor, or alternatively reducing themotor volume by 20% compared to the conventional motor while increasingthe torque properties twofold, thus greatly increasing the efficiency.

[0021] Moreover, when this anisotropic rare earth bonded magnet isformed by resin molding, it is easy to achieve precision forming.Because of this, the permanent magnet for the inside of the motorhousing can be formed into a precise hollow cylinder shape. With this,it is possible to have precise rotational symmetry of the magnetic fieldinside of the motor made by the permanent magnet. When the interiormagnetic field has a high degree of symmetry, the center electromagneticrotor core receives uniform torque and can rotate. Consequently, themotor is a rather quiet motor, without the rattle and squeak of theconventional motor caused by uneven torque. Furthermore, the use of ahollow-cylinder-shaped resin-formed anisotropic rare earth bonded magnetmakes the assembly of the motor housing simple. There is no need toassembly each discrete sintered ferrite magnet of the 2-pole or 4-polemotor as in the conventional motor. Thus, it also has the advantage ofsimplifying the manufacturing process.

[0022] Additionally, the anisotropic rare earth bonded magnets used aspermanent magnets in the motors of the second aspect have a maximumenergy product no less than 14 MGOe.

[0023] Compared to sintered ferrite magnets, anisotropic rare earthbonded magnets that have superior characteristics and a maximum energyproduct no less than 14 MGOe are much preferred.

[0024] Moreover, the motor as mentioned in the third aspect has thespecial characteristics that for a motor housing outer diameter r, ananisotropic rare earth bonded magnet radial thickness d, anelectromagnetic rotor core radius a, and a motor housing thickness w,the ratio of electromagnetic rotor core radius to housing outer diametera/r is not less than 0.25 and not greater than 0.5, the ratio of housingthickness to magnet thickness w/d is not less than 1 and not greaterthan 4, and the ratio of magnet thickness to housing outer diameter d/ris not less than 0.01 and not greater than 0.10.

[0025] The abovementioned motor housing is intended to include the backyoke, and the motor housing outer diameter r has the meaning of theouter diameter of the motor including the back yoke.

[0026] The range limitation of the ratio of electromagnetic rotor coreradius to housing outer diameter a/r given here is the common-senserange for commonly used DC brush motors. When a/r is less than 0.25, theelectromagnetic rotor core is notably small compared to the motorhousing, and from the point of view of motor properties it is clear thatthe design of the magnet and housing is wasteful. Therefore a/r isgenerally kept no less than 0.25.

[0027] When a/r is 0.5, the motor housing outer diameter and theelectromagnetic rotor core diameter (2 a) are equal, so it is obviousthat a/r must be less than 0.5.

[0028] The ratio of housing thickness to magnet thickness w/d is keptwithin the range of 1 to 4 for the following reasons. In the case of aDC brush motor using ferrite magnets, because the magnetic force of themagnet is weak, it is possible have design which sufficiently preventsmagnetic leakage even with a thin housing thickness compared to magnetthickness. However, when using an anisotropic rare earth bonded magnet,the ratio w/d must be no less than 1 because when it is less than 1 themagnetic leakage cannot be prevented due to the strong magnetic force ofthe magnet. When the ratio w/d is greater than 4, even with magnetshaving a strong magnet force the housing thickness becomes too thick;there is no magnetic leakage but there is additional meaningless housingthickness, thus preventing sufficient size reduction and causing adeterioration of the motor properties.

[0029] The range of the ratio of magnet thickness to housing outerdiameter d/r was decided based on the following. Permanent magnetattractive strength increases according to magnet thickness. When theratio of magnet thickness to housing outer diameter d/r is less than0.01, the demagnetizing field becomes large and the magnetic attractivestrength drops off rapidly, and therefore the prescribed torque can notbe obtained. Therefore, it is best to keep the ratio of magnet thicknessto housing outer diameter d/r no less than 0.01.

[0030] If for example you want to increase a motor's performance index T(T=torque constant/volume) to twice that of the conventional motor, thatis to say to obtain a T of 2.6 which is twice that of the conventional2-pole ferrite motor where T equals approximately 1.3, it is necessaryto make the ratio of magnet thickness to housing outer diameter d/r lessthan 0.1. Consequently, it is desirable to keep the ratio of magnetthickness to housing outer diameter d/r no less than 0.01 and notgreater than 0.10. It is certainly possible to realize the motors asmentioned in the first and the second aspects in this way.

[0031] Moreover, the motor as mentioned in the fourth aspect has thespecial characteristics that for motor housing outer diameter r, theanisotropic rare earth bonded magnet radial thickness d, theelectromagnetic rotor core radius a, and the motor housing thickness w,the ratio of electromagnetic rotor core radius to housing outer diametera/r is not less than 0.25 and not greater than 0.5, the ratio of housingthickness to magnet thickness w/d is not less than 1 and not greaterthan 4, and the ratio of magnet thickness to housing outer diameter d/ris not less than 0.01 and not greater than 0.08.

[0032] At a ratio of magnet thickness to housing outer diameter d/r of0.08, the motor performance index T per unit quantity of magnet used(the motor performance index T/quantity magnet used, or the ratio S, ishereafter referred to as “magnet efficiency”) is equal to the magnetefficiency times the magnet performance multiple m of the conventional2-pole ferrite motor. When the ratio of magnet thickness to housingouter diameter d/r is less than or equal to 0.08, the magnet efficiencyS of the motor of the present invention is not less than the magnetefficiency times the magnet performance multiple m of the conventional2-pole ferrite motor. However, when the ratio of magnet thickness tohousing outer diameter d/r is smaller than its lower limit of 0.01, asstated above, the demagnetizing field becomes large and the magneticattractive strength drops off rapidly, and the prescribed torque can notbe obtained. Therefore it is desired to keep the ratio of magnetthickness to housing outer diameter d/r not less than 0.01. In this waywhen the ratio of magnet thickness to housing outer diameter d/r is notless than 0.01 and not greater than 0.08, the magnet efficiency S is notless than the magnet efficiency times the magnet performance multiple mof the conventional 2-pole ferrite motor. Here, the magnet performancemultiple m is defined as the anisotropic bonded magnet performance[(BH)max] divided by the ferrite sintered magnet performance [(BH) max].For example, if the performance (maximum energy product) of ananisotropic bonded magnet is 14 MGOe, and the performance (maximumenergy product) of a ferrite sintered magnet is 3.5, then the magnetperformance multiple m is 4. Furthermore, when the magnet efficiency Sis equal to the magnet efficiency times the magnet performance multiplem of the conventional 2-pole ferrite motor, the ratio of magnetthickness to housing outer diameter d/r is approximately equal to 0.08if the anisotropic bonded magnet's maximum energy product is no lessthan 14 MGOe.

[0033] Moreover, the motor as mentioned in the fifth aspect has thespecial characteristics that for motor housing outer diameter r, theanisotropic rare earth bonded magnet radial thickness d, theelectromagnetic rotor core radius a, and the motor housing thickness w,the ratio of electromagnetic rotor core radius to housing outer diametera/r is not less than 0.25 and not greater than 0.5, the ratio of housingthickness to magnet thickness w/d is not less than 1 and not greaterthan 4, and the ratio of magnet thickness to housing outer diameter d/ris not less than 0.01 and not greater than 0.05.

[0034] When the ratio of magnet thickness to housing outer diameter d/ris less than or equal to 0.05, the magnet efficiency S is not less thantwo times greater than when d/r is 0.08. That is to say that at a ratioof magnet thickness to housing outer diameter d/r of 0.05, the magnetefficiency S of the motor of the present invention is equal to two timesthe magnet efficiency times the magnet performance multiple m of theconventional 2-pole ferrite motor. When the ratio of magnet thickness tohousing outer diameter d/r is less than or equal to 0.05, the magnetefficiency S of the motor of the present invention is not less than twotimes the magnet efficiency times the magnet performance multiple m ofthe conventional 2-pole ferrite motor.

[0035] For example, when the performance (maximum energy product) of theanisotropic bonded magnet is 14 MGOe, and the performance (maximumenergy product) of the ferrite sintered magnet is 3.5 MGOe, the magnetperformance multiple m is 4. When the magnets have these figures, themagnet efficiency S is not less than 8 times the magnet efficiency ofthe conventional 2-pole ferrite motor. Therefore, it is much desiredthat the figures be kept within this range.

[0036] Moreover, the motor as mentioned in the sixth aspect has thespecial characteristics that for motor housing outer diameter r, theanisotropic rare earth bonded magnet radial thickness d, theelectromagnetic rotor core radius a, and the motor housing thickness w,the ratio of electromagnetic rotor core radius to housing outer diametera/r is not less than 0.25 and not greater than 0.5, the ratio of housingthickness to magnet thickness w/d is not less than 1 and not greaterthan 4, and the ratio of magnet thickness to housing outer diameter d/ris not less than 0.02 and not greater than 0.05.

[0037] With regards to the magnet efficiency S, it is similar to thesituation of the fifth aspect where when the ratio of magnet thicknessto housing outer diameter d/r is less than or equal to 0.05, the magnetefficiency S is not less than twice that when d/r is 0.08, which is tosay that the magnet efficiency S is greater than or equal to two timesthe magnet efficiency times the magnet performance multiple m of theconventional 2-pole ferrite motor. The motor performance index T is nearits maximum value when the ratio of magnet thickness to housing outerdiameter d/r is not less than 0.02 and not greater than 0.05. When themaximum energy product is 14 MGOe, a motor performance index T of themotor of the present invention that is 2.3 times that of the motorperformance index T of the conventional 2-pole ferrite motor isobtained. When the maximum energy product is 17 MGOe, a motorperformance index T of the motor of the present invention that is 2.5that of the motor performance index T of the conventional 2-pole ferritemotor is obtained. When the maximum energy product is 25 MGOe, a motorperformance index T of the motor of the present invention that is 2.6times that of the motor performance index T of the conventional 2-poleferrite motor is obtained. Thus, a ratio of magnet thickness to housingouter diameter d/r in the range not less than 0.02 and not greater than0.05 is much desired from the viewpoints of motor performance index Tand magnet efficiency S.

[0038] The permanent magnet as mentioned in the seventh aspect has thespecial characteristics that it is a permanent magnet located on theperiphery of the electromagnetic rotor core of a DC brush motor, and isan anisotropic rare earth bonded magnet in a thin-walled hollow cylindershape magnetized with at least 4 poles. This anisotropic rare earthbonded magnet is for example a magnet manufactured via resin forming ofNdFeB-based magnet powder, and strongly magnetized along one axis. Thismagnet has the special characteristic of having a maximum energy product(BHmax) not less than four times greater than that of the conventionalsintered ferrite magnet.

[0039] After an investigation by the inventers of the present inventioninto how the potential of these anisotropic bonded magnets could beharnessed, they found that there would be remarkable advantages inmaking these magnets thin and using them in 1-300 W small-sized brushmotors in particular. They found that at the same time as greatlyreducing the length of the magnetic path the magnetic circuit of eachmagnetic pole by making four or more magnetic poles, the motor volumecan be reduced by ½ compared to the conventional motor while keeping thetorque properties the same, thus resulting in a considerably smaller,light-weight motor that could not have been imagined previously, oralternatively the motor volume can be reduced by 20% compared to theconventional motor while increasing the torque properties twofold, thusgreatly increasing the efficiency.

[0040] As this anisotropic rare earth bonded magnet is made by resinforming, its shape can be formed with good precision. Because of this,it is possible to make a very precise hollow cylinder shaped permanentmagnet for the inside of the magnet housing. With the adoption of such amagnet, it is possible to obtain precise rotational symmetry of themotor interior's magnetic field. An interior field with a high degree ofsymmetry enables the electromagnetic rotor core in the center to receiveuniform torque. By adopting this magnet, the noise associated withuneven torque can be reduced, and a quiet motor can be realized. Inaddition, as the anisotropic rare earth bonded magnet can be resinformed into a precise hollow cylinder shape, the assembly into the motorhousing becomes easier. There is no need to assemble each of thediscrete 2-pole or 4-pole sintered ferrite magnets as has been necessaryup until now. This anisotropic rare earth bonded magnet also has theadvantage of making the motor manufacturing process simpler.

[0041] The permanent magnet mentioned in the eighth aspect have thespecial characteristic that they are an anisotropic rare earth bondedmagnet with a maximum energy product not less than 14 MGOe.

[0042] Anisotropic rare earth bonded magnets are preferred to sinteredferrite magnets as they have superior magnetic characteristics and havea maximum energy product not less than 14 MGOe.

[0043] Moreover, when an anisotropic rare earth bonded magnet with amaximum energy product not less than 14 MGOe is prepared with 4 or moremagnetic poles as mentioned in the seventh aspect, the torque output isvery high. Therefore, if the same torque conditions are adopted, theaxial length of the anisotropic rare earth bonded magnet can be cutback,and the motor volume can be further reduced. For example, as will beexplained in the following text, the volume of the conventional motorusing a sintered ferrite magnet can be reduced by roughly 50%.

[0044] Moreover, the permanent magnet as mentioned in the ninth aspecthas the special characteristics that for motor housing outer diameter r,the anisotropic rare earth bonded magnet radial thickness d, theelectromagnetic rotor core radius a, and the motor housing thickness w,the ratio of electromagnetic rotor core radius to housing outer diametera/r is not less than 0.25 and not greater than 0.5, the ratio of housingthickness to magnet thickness w/d is not less than 1 and not greaterthan 4, and the ratio of magnet thickness to housing outer diameter d/ris not less than 0.01 and not greater than 0.10.

[0045] The range of a/r and the range of w/d have the same values asdiscussed in the explanation of the invention of the third aspect.

[0046] The magnetic force of a permanent magnet is proportional to itsthickness. When the ratio of magnet thickness to housing outer diameterd/r is less than 0.01, the demagnetizing field becomes large and theresultant magnetic force quickly deteriorates. A motor created withthese conditions could not obtain the prescribed output. Therefore whenpermanent magnets are to be used in a motor, it is desired that theratio of anisotropic rare earth bonded magnet thickness to housing outerdiameter d/r is not less than 0.01.

[0047] Using this anisotropic rare earth bonded magnet, in order todouble the conventional motor performance index T (T=torqueconstant/volume), for example to obtain T=2.6 that is double theconventional 2-pole ferrite motor performance index (around 1.3), theratio of magnet thickness to housing outer diameter d/r must be lessthan or equal to 0.1. The conditions are, for example, those of thesituation whose description follows where the interior electromagneticrotor core diameter is kept the same. Thus it is desirable to keep theratio of magnet thickness to housing outer diameter d/r not less than0.01 and not greater than 0.10. If these conditions are combined withthe permanent magnet mentioned in the fourth and fifth aspects, it issure that a motor can be realized which is quieter than the conventionalmotor and which has ½ the volume with the same torque, or which hastwice the torque with a 20% reduced volume.

[0048] Moreover, the permanent magnet as mentioned in the tenth aspecthas the special characteristics that for motor housing outer diameter r,anisotropic rare earth bonded magnet radial thickness d, electromagneticrotor core radius a, and motor housing thickness w, the ratio ofelectromagnetic rotor core radius to housing outer diameter a/r is notless than 0.25 and not greater than 0.5, the ratio of housing thicknessto magnet thickness w/d is not less than 1 and not greater than 4, andthe ratio of magnet thickness to housing outer diameter d/r is not lessthan 0.01 and not greater than 0.08.

[0049] The values of these figures are the same as those explained inthe invention of the fourth aspect.

[0050] Moreover, the permanent magnet as mentioned in the eleventhaspect has the special characteristics that for motor housing outerdiameter r, anisotropic rare earth bonded magnet radial thickness d,electromagnetic rotor core radius a, and motor housing thickness w, theratio of electromagnetic rotor core radius to housing outer diameter a/ris not less than 0.25 and not greater than 0.5, the ratio of housingthickness to magnet thickness w/d is not less than 1 and not greaterthan 4, and the ratio of magnet thickness to housing outer diameter d/ris not less than 0.01 and not greater than 0.05.

[0051] The values of these figures are the same as those explained inthe invention of the fifth aspect.

[0052] Moreover, the permanent magnet as mentioned in twelfth aspect hasthe special characteristics that for motor housing outer diameter r,anisotropic rare earth bonded magnet radial thickness d, electromagneticrotor core radius a, and motor housing thickness w, the ratio ofelectromagnetic rotor core radius to housing outer diameter a/r is notless than 0.25 and not greater than 0.5, the ratio of housing thicknessto magnet thickness w/d is not less than 1 and not greater than 4, andthe ratio of magnet thickness to housing outer diameter d/r is not lessthan 0.02 and not greater than 0.05.

[0053] The values of these figures are the same as those explained inthe invention of the sixth aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Various other objects, features, and many of the attendantadvantages of the present invention will be readily appreciated as thesame becomes better understood with reference to the following detaileddescription of the preferred embodiments when considered in connectionwith the accompanying drawings, in which:

[0055]FIG. 1 is a configuration of a motor of a concrete embodiment ofthe present invention;

[0056]FIG. 2 is a configuration of a motor of another concreteembodiment of the present invention;

[0057]FIG. 3 is characteristics of the relation between number of polesand performance index for motors using ferrite sintered magnets andmotors using an anisotropic bonded magnet;

[0058]FIG. 4 is characteristics of the relation between performanceindex T and ratio R, for an anisotropic rare earth bonded magnet with amaximum energy product of 14 MGOe;

[0059]FIG. 5 is characteristics of the relation between magnetefficiency S and ratio R, for an anisotropic rare earth bonded magnetwith a maximum energy product of 14 MGOe;

[0060]FIG. 6 is characteristics of the relation between performanceindex T and ratio R, for an anisotropic rare earth bonded magnet with amaximum energy product of 17 MGOe;

[0061]FIG. 7 is characteristics of the relation between magnetefficiency S and ratio R, for an anisotropic rare earth bonded magnetwith a maximum energy product of 17 MGOe;

[0062]FIG. 8 is characteristics of the relation between performanceindex T and ratio R, for an anisotropic rare earth bonded magnet with amaximum energy product of 25 MGOe;

[0063]FIG. 9 is characteristics of the relation between magnetefficiency S and ratio R, for an anisotropic rare earth bonded magnetwith a maximum energy product of 25 MGOe;

[0064]FIG. 10 is configuration of brush location for the motor ofembodiment 5; and

[0065]FIG. 11 is configuration of coils for the motor of embodiment 5.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0066] The following will be an explanation of the implementation of thepresent invention. However, the present invention is not limited to thefollowing implementation.

Embodiment 1

[0067]FIGS. 1A and 1B shows an example motor of the present embodiment.This figure includes the side view 1A and the cross sectional view 1Bthrough AA′. The purpose of the present embodiment is to make a smallermotor than the conventional motor. The motor of the present embodimentis comprised of a housing 12, an anisotropic rare earth bonded magnet 13as the hollow cylinder permanent magnet set in the inner perimeter ofthe housing 12, an armature 14 making the electromagnetic rotor core setin the center, coils 15 wrapped around armature 14, a rotary shaft 11extending from the center of armature 14, and a back yoke 10 that is aflux ring for prevention of magnetic flux leakage. The back yoke is apart of the housing. The motor housing and the back yoke have the samefunction as a magnetic circuit. Therefore In the present embodiment, thehousing outer diameter as mentioned in the claims is the diameter of theback yoke. For a volume comparison, the conventional 2-pole motor isshown in FIGS. 1C and 1D. This figure includes the side view 1C and thecross sectional view lD through AA′. For the sake of comparison of bothmotors, the armature 14 outer diameter is the same. In recent years ithas become possible to mass-produce said anisotropic rare earth bondedmagnet 13, although this depends on the applicant. For example, thisanisotropic rare earth bonded magnet 13 can be made by the manufacturingprocess as laid out in Published Unexamined Patent Application Number2001-7691A, U.S. Pat. No. 2,816,668 and U.S. Pat. No. 3,060,104. Theseanisotropic rare earth bonded magnets with a maximum energy product of14 MGOe - 25 MGOe can presently be manufactured.

[0068] The difference between the motor of the present embodiment (FIGS.1A and 1B) and the conventional motor (FIGS. 1C and 1D) is that thesintered ferrite magnets 23 used as the permanent magnets of theconventional motor have been replaced by a NdFeB-based hollow cylinderanisotropic rare earth bonded magnet 13. This magnet has been magnetizedwith 4 poles, and the magnet path length of each pole's magnetic circuithas been greatly reduced, thus allowing for an increase of torqueapplied to the armature. This is the first distinctive feature of thepresent invention. Anisotropic rare earth bonded magnet 13 is a magnetthat is manufactured via resin forming of NdFeB-based magnet powder, andis strongly magnetized in the axial direction. The material of theanisotropic rare earth bonded magnet may be NdFeB or a NdFeB-typematerial, for example a combination of Nd and a rare earth element otherthan Nd, or a material with other additive elements. Furthermore,materials containing rare earth elements other than Nd, such asSmFeN-type materials, SmCo-type materials, NdFeB-type materials or acombination of these materials, may also be used. Bonded magnets arealso called plastic magnets. This magnet has the special characteristicthat its maximum energy product (BHmax) is not less than four timesgreater than that of the conventional sintered ferrite magnet. That isto say that it has a maximum energy product not less than 14 MGOe, whichis around 4 times the maximum energy product of 3.5 MGOe of the standardsintered ferrite magnet 23. This means that if the motor torque (sametorque conditions) is kept the same as for the conventional motor, it ispossible to reduce the thickness of the permanent magnet to ¼.

[0069] The second distinctive feature is that when the permanent magnethas been scaled down, for motor housing (back yoke 10) outer diameter(housing outer diameter) r, hollow cylinder anisotropic rare earthbonded magnet 13 thickness (magnet thickness) d, electromagnetic rotorcore that is armature 14 radius (electromagnetic rotor core radius) a,motor housing thickness (thickness including the housing 12 and the backyoke 10) (housing thickness) w, the ratio of electromagnetic rotor coreradius to housing outer diameter a/r is not less than 0.25 and notgreater than 0.5, the ratio of housing thickness to magnet thickness w/dis not less than 1 and not greater than 4, and the ratio of magnetthickness to housing outer diameter d/r is not less than 0.01 and notgreater than 0.10 (under small sized conditions). The values for theranges of a/r and w/d are the same as those mentioned in the explanationof the invention of the third aspect under the mechanism of solving saidchallenges as well as effects of using the present invention.

[0070] The lower limit of the ratio of magnet thickness to housing outerdiameter was decided to be 0.01 because at values less than that thedemagnetizing field rapidly increases, resulting in a decrease inmagnetic force, and thus the prescribed motor torque is unable to beobtained. The upper limit of the ratio of magnet thickness to housingouter diameter d/r is the condition under which the motor performanceindex T (T=torque constant/volume) is twice that of the conventionalmotor. That is to say, it is the condition under which the volume can bereduced by ½ or the torque can be multiplied by two. For example, if theperformance index is approximately 1.3 times that of the conventional2-pole ferrite motor, then the condition is that at which theperformance index T is equal to 2.6. Under these conditions, a smallmotor with 50% reduced volume but the same torque can be realized.

[0071] For example, a conventional motor using sintered ferrite magnet23 has a back yoke 10 (motor housing) outer diameter of 38 mm, an innerdiameter of 32 mm, a motor output (torque) of 75.7 (mN*m/A), and avolume of approximately 56.1 cm³. The sintered ferrite magnet 23 has anouter diameter of 32 mm, inner diameter of 24, and a radial length(thickness) of approximately 4 mm. Accordingly, a/r=0.30, w/d=0.75 andd/r=0.11.

[0072] At the same time, the motor of the present embodiment, which hasthe same torque, has a back yoke 10 of outer diameter r=31 mm and innerdiameter of 26 mm, and a volume of 24.5 cm³. Anisotropic rare earthbonded magnet 13 has an outer diameter of 26 mm, an inner diameter of 24mm and a radial thickness of d=1 mm, and it is magnetized in a 4-poleconfiguration. With 4-pole magnetization, the magnetic path length ofthe magnetic circuit is shortened. Therefore, a/r=0.37, w/d=2.5 andd/r=0.03. Thus when instituted, a motor whose volume is 44% of thevolume of a conventional motor, while maintaining the same torque, canbe realized. Moreover, the performance index T is 3.09, which is 2.3times that of the conventional motor's 1.35.

[0073] In the present embodiment, the armature thickness was decided sothat the torque would be the same as that of the conventional motor.This was done because the anisotropic rare earth bonded magnet 13 of thepresent embodiment was magnetized in a 4-pole configuration. Theconventional armature thickness is approximately 17.5 mm, while thepresent embodiment's armature thickness is approximately 9.8 mm. Themotor's axial length was decided to include the part at the rear of themotor shared by the commutator. By doing this, the axial length of theconventional motor L_(F) is approximately 50 mm, while the axial lengthof the motor of the present embodiment is approximately 33 mm, thusmaking the length reduction ratio L_(N)/L_(F)=0.66. Moreover, the weightof the conventional motor is 245 g, while that of the motor of thepresent embodiment is 191 g, thus indicating a decrease of 49% comparedto the conventional.

[0074] Moreover, as the anisotropic rare earth bonded magnet 13 of thepresent practical invention is manufactured by resin forming, it can bemade into a precise hollow cylinder shape. Therefore, anisotropic rareearth bonded magnet 13 can easily be precisely and symmetricallymagnetized. Because the magnetic field in the inner part of the motorcan be generated precisely and symmetrically, armature 14 will receiveuniform torque. Thus the squeaking and rattling associated with theconventional motor during rotation does not occur, and a quiet motor canbe achieved.

[0075] The sintered ferrite 2-pole motor and the anisotropic rare earthbonded 4-pole motor were mentioned above and their respectivedescriptions were given. The sintered ferrite 4-pole motor andanisotropic rare earth bonded 2-pole motor are shown in Table 1 forcomparison. TABLE 1 Magnet Type Ferrite Ferrite Ferrite AnisotropicAnisotropic Anisotropic Magnet (sintered, bonded) Sintered SinteredSintered Bonded Bonded Bonded Maximum Energy Product 3.5 MGOe 3.5 MGOe3.5 MGOe 17 MGOe 17 MGOe 17 MGOe Number of Magnetic Poles 2 4 6 2 4 6Magnet Size Diameter 32-24 32-24 32-24 28-24 26-24 25.4-24 (mm)Thickness 4 4 4 2 1 0.7 (mm) Length 21 18 18 12.25 11.3 12.42 (mm) 135deg. 67.5 deg. 45 deg. ring ring ring 2 tiles 4 tiles 6 tiles Back YokeDiamter 38-32 38-32 38-32 34-28 31-26 28.4-25.4 (mm) Thickness 3 3 3 32.5 1.5 (mm) Length 49.5 46.5 46.5 42.75 33 35.12 (mm) ArmatureThickness (mm) 17.5 15 15 10.75 9.8 10.8 Torque Constant (mN*m/A) 75.775.7 75.7 75.7 75.7 75.7 Motor Volume (cm³) 56.13 52.7 52.7 38.8 24.522.25 Motor Weight (g) 245 229 229 148 119 100 Performance Index T 1.351.44 1.44 1.95 3.09 3.40

[0076] The housing outer diameter of the present embodiment is the sameas the back yoke outer diameter.

[0077] Back yoke material SPCC

[0078] Armature material, dimensions silicone sheet, ø23 mm

[0079] Coil winding, turns 145 turns

[0080] Amount of Current 1 A

[0081] Fixed Conditions Armature outer diameterø23 mm was kept the same,and axial thickness was adjusted to achieve the same torque.

[0082] In order to explain the results of the abovementioned figure inan easy to understand way, the relation between the performance indexand the number of poles is shown in FIG. 3. From FIG. 3 it can be seenthat there is no major improvement when the magnet of a conventionalsintered ferrite 2-pole motor is simply replaced with an anisotropicrare earth bonded magnet to make an anisotropic rare earth bonded 2-polemotor, nor when a 2-pole sintered ferrite motor is simply changed to a4-pole sintered ferrite motor. The performance index remains in the1.2-2.0 range. But compared to this, as is presented in the presentembodiment, when an anisotropic rare earth bonded magnet is used incombination with the switch to a 4-pole motor, a performance index of3.09 can be obtained. This figure indicates the realization of asignificant improvement over the performance index of the conventionalsintered ferrite 2-pole motor with a performance index increase of 2.3times.

Embodiment 2

[0083] Embodiment 1 was an example of the size reduction of theconventional motor by the use of an anisotropic rare earth bondedmagnet. Using this anisotropic rare earth bonded magnet, with adjustmentof radial thickness, it is possible to increase the motor torque. Thepresent embodiment is an example of using an anisotropic rare earthbonded magnet to double the torque.

[0084] For example, the conventional motor using sintered ferrite magnet23 has a torque of 75.7 (mN*m/A), a volume of approximately 56.1 cm³,which is to say a motor performance index T of T=1.35, and all otherdimensions the same as those of the conventional motor explained inembodiment 1.

[0085] The motor of the present embodiment is shown in FIG. 2. In thepresent embodiment, the ratio of electromagnetic rotor core radius tohousing outer diameter a/r is not less than 0.25 and not greater than0.5, the ratio of housing thickness to magnet thickness w/d is not lessthan 1 and not greater than 4, and the abovementioned conditions forsmall size (0.01≦d/r≦0.1) are adhered to. The motor housing (back yoke10) outer diameter r=34 mm, and inner diameter is 28 mm. Anisotropicrare earth bonded magnet 13 is magnetized with 4 poles, and has an outerdiameter of 28 mm, an inner diameter of 24 mm, and a thickness of 2 mm.In this case, a/r=0.34, w/d=1.5 and d/r=0.06. With the 4-pole magneticconfiguration, the magnetic path length of the magnetic circuit isshortened. The armature thickness is 17.5 mm, which is the same as thatof the conventional motor. The motor of the present embodiment isdesigned to achieve twice the torque (155.5 mN*m/A) of the conventionalmotor. In addition, at 41.2 cm³ the volume of this motor realizes a 27%volume reduction ratio compared to the conventional motor, and theweight of the motor of the present embodiment is 185 g, which is a 76%reduction compared to the 245 g of the conventional motor.

[0086] Similarly to embodiment 1, in this case anisotropic rare earthbonded magnet 13 can be precisely formed into a hollow-cylinder shape,and very symmetrical magnetic field can be generated. Thus a highoutput, quiet motor can be achieved.

Embodiment 3

[0087] The motor of embodiment 1, which is a low output level DC brushmotor of the present invention, is maintained with the followingcommon-sense conditions (1) the ratio of electromagnetic rotor coreradius to housing outer diameter a/r is not less than 0.25 and notgreater than 0.5, and (2) the ratio of housing thickness to magnetthickness w/d is not less than 1 and not greater than 4, while the ratioof anisotropic rare earth bonded magnet 12 thickness to housing outerdiameter d/r=R (hereafter referred to simply as ratio R) is changed andthe motor performance index T is evaluated. The characteristics whenanisotropic rare earth bonded magnet 12 has a maximum energy product of14 MGOe are shown in FIG. 4. When ratio R is in the range not less than0.01 and not greater than 0.10, the performance index T is greater thantwice the performance index T of the conventional 2-pole ferrite motor(1.3). If the ratio R is less than the lower limit of 0.01, even withmagnet strength of 25 MGOe, the superior characteristics of twice theperformance index of the abovementioned conventional motor will not beobtained.

[0088] Similarly, the characteristics for anisotropic rare earth bondedmagnet 12 with maximum energy product of 17 MGOe and 25 MGOe are shownin FIGS. 6 and 8 respectively. It can be understood that when themaximum energy product is larger, the overall performance index T islarger as well.

[0089] Next, where the volume of anisotropic rare earth bonded magnet 12is v, and the performance index T per bonded magnet unit volume T/v isthe magnet efficiency S, the variation characteristics with relation toratio R were investigated. The characteristics when the maximum energyproduct was 14 MGOe, 17 MGOe and 25 MGOe are shown in FIGS. 5, 7 and 9respectively. When ratio R is not less than 0.01 and not greater than0.08, it can be understood that the magnet efficiency S is not less thanthe magnet efficiency of the conventional 2-pole ferrite motor times themagnet performance multiple m. These characteristics can be realized formaximum energy products not less than 14 MGOe.

[0090] When the ratio R is less than or equal to 0.05, the magnetefficiency S is at least twice that when R is 0.08. That is to say themagnet efficiency S is not less than two times the magnet efficiency ofthe conventional 2-pole ferrite motor times the magnet performancemultiple m. This means that it is equal to the magnet efficiencymultiplied by twice the magnet performance multiple m. In this case, themagnet performance multiple m is twice as efficient as sintered ferrite,and it is possible to increase the motor performance index T per unitamount of magnet used. These characteristics can be realized for maximumenergy products not less than 14 MGOe.

[0091] When ratio R is in the range not less than 0.02 and not greaterthan 0.05, the magnet efficiency S is at least twice that when d/r is0.08. That is to say that the magnet efficiency S is not less than 2 mtimes greater than the magnet efficiency of the conventional 2-poleferrite motor. When the motor performance index T is evaluated, it canbe understood that when the ratio of magnet thickness to housing outerdiameter d/r is in the range not less than 0.02 and not greater than0.05 the motor performance index T is at about its maximum value. With amaximum energy product of 14 MGOe, a performance index T that is 2.3times that of the conventional 2-pole ferrite motor can be obtained.With a maximum energy product of 17 MGOe, a performance index 2.5 timesthat of the conventional 2-pole ferrite motor can be obtained. With amaximum energy product of 25 MGOe, a performance index 2.6 times that ofthe conventional 2-pole ferrite motor can be obtained. Thesecharacteristics can be realized for maximum energy products not lessthan 14 MGOe.

[0092] Therefore, from the viewpoints of both motor performance index Tand magnet efficiency S, it is desired to keep the ratio of magnetthickness to housing outer diameter d/r in the range not less than 0.02and not greater than 0.05.

[0093] The magnet efficiency S is thought of in the following way. Wheretorque constant is τmotor volume is V, anisotropic rare earth bondedmagnet volume is v, motor outer diameter is r, anisotropic bonded magnetradial thickness is d, ratio of magnet thickness to housing outerdiameter d/r is R, electromagnetic rotor core radius is a, housingthickness is w, motor effective length is L, and air gap between theelectromagnetic rotor core and the anisotropic rare earth bonded magnetis neglected, the following equations can be written.

[0094] [Equation 1]

2a+2d+2w=r   (1)

[0095] [Equation 2]

R=d/r   (2)

[0096] [Equation 3]

V=πr ² L/4   (3)

[0097] [Equation 4]

v=π{(a+d)² −a ² }L   (4)

Because d <<a,

[0098] [Equation 5]

v=2πadL   (5)

[0099] Therefore magnet efficiency S follows the following equation,$\begin{matrix}\left\lbrack {{Equation}\quad 6} \right\rbrack & \quad \\{S = {\tau/({Vv})}} & (6) \\{\quad {= {2{\tau/\left\{ {\pi^{2}L^{2}a\quad d\quad r^{2}} \right\}}}}} & \quad \\{\quad {= {2\quad {\tau/\left\{ {\pi^{2}{L^{2}\left( {{r/2} - d - w} \right)}d\quad r^{2}} \right\}}}}} & \quad\end{matrix}$

[0100] When d=Rr is substituted into (6), the following equation isobtained. $\begin{matrix}\left\lbrack {{Equation}\quad 7} \right\rbrack & \quad \\{S = {2\quad {\tau/\left\{ {\pi^{2}\quad {L^{2}\left( {{r/2} - {Rr} - w} \right)}\quad R\quad r^{3}} \right\}}}} & (7) \\{\quad {= {\tau/\left\{ {\pi^{2}L^{2}\quad {r^{3}\left\lbrack {{\left( {1 - {2R}} \right)\quad r} - {2w}} \right\rbrack}\quad R} \right\}}}} & \quad\end{matrix}$

[0101] These characteristics are shown in FIGS. 5, 7 and 9.

Embodiment 4

[0102] A 6-pole motor with the same dimensions as embodiment 1 wasmanufactured. This motor's dimensions and characteristics are as shownin Table 1. Similarly, the performance index T was evaluated withrespect to a 6-pole motor using a ferrite magnet. This motor'sdimensions and characteristics are also shown in Table 1. Performanceindex T characteristics were obtained as shown in FIG. 3. It can beunderstood from these characteristics that when increasing from a 2-poleto 4-pole design, the performance index T of the present invention'smotor that uses an anisotropic rare earth bonded magnet drasticallyincreases. Furthermore, when increasing from a 4-pole to 6-pole design,the performance index T is 1.10 times that of a 4-pole and 1.74 timesthat of a 2-pole. There is not much variation in the motor performanceindex T for the various pole configurations of a motor using a ferritemagnet. That is to say, with an increase from a 4-pole to a 6-poledesign there is no change at all from the 4-pole performance index T,and the performance index T of the 6-pole design stops at 1.07 timesgreater than the performance index for a 2-pole design. From this it canbe understood that the motor of the present invention using ananisotropic rare earth bonded magnet makes it possible to obtain resultsthat were thought to be impossible with the conventional motor using aferrite magnet.

Embodiment 5

[0103] In the motor of embodiment 1, brush 30 a and 30 b are arranged asshown in FIG. 10. That is to say that the brushes are not facing eachother at 180°, but rather are set at a position 90° from each other.With this arrangement, there is a space in which no brush exists, shownby area Q in FIG. 10. Because this space is made to be so large, anelectrical circuit can be arranged in this area Q. In a 6-pole design,the two brushes will be set 60° apart and the large space can bemaintained. In an 8-pole design, the two brushes can be set 22.5° or67.5° apart and the large space can be maintained.

[0104] When a motor with this kind of 2-pole brush configuration isused, coils like those in FIG. 11 are one example for a 4-poled motor.

MODIFIED EXAMPLE

[0105] The abovementioned embodiments are one group of examples ofpractical forms of the present invention, but many other modifiedexamples can be thought of. For example, in the abovementionedembodiment the anisotropic rare earth bonded magnet 13 was magnetized ina 4-pole configuration, but greater than 4 poles is also acceptable. Forexample, 6poles or8poles are acceptable. If the number of magnetic polesis increased, the magnetic path length gets shorter and therefore themagnetic flux across the armature coils is increased. Moreover, becauseit is possible to easily magnetize anisotropic rare earth bonded magnet13, a higher power, quiet motor can be realized.

[0106] Moreover, in the abovementioned embodiment, the anisotropic rareearth bonded magnet 13 is made by resin forming, but it is alsoacceptable to further process the magnet after resin forming viatrimming, etc. for higher precision. With increased dimension precision,a quiet motor without uneven torque is possible.

[0107] The present invention has been described in detail with referenceto the above embodiments serving as most practical and appropriateexamples. However, the present invention is not limited to theseembodiments, and appropriate modifications and applications can be madewithout deviating from the scope of the present invention.

1-20. (Cancelled)
 21. A DC brush motor with a superior motor performanceindex (torque constant/motor volume) comprising: a permanent magnet,arranged on an inner perimeter of a housing, the permanent magnet beingformed as a thin-walled hollow cylinder shaped anisotropic rare earthbonded magnet having a maximum energy product greater than 14 MGOe andmagnetized with at least 4 magnetic poles; and an electromagnetic rotorcore arranged in the center of the motor.
 22. A DC brush motor accordingto claim 21, wherein output power of said DC brush motor is in a rangefrom 1 to 300 W.
 23. A DC brush motor with a superior motor performanceindex (torque constant/motor volume) comprising: a permanent magnet,arranged on an inner perimeter of a housing, the permanent magnet beingformed as a thin-walled hollow cylinder shaped anisotropic rare earthbonded magnet having a maximum energy product greater than 17 MGOe andmagnetized with at least 4 magnetic poles; and an electromagnetic rotorcore arranged in the center of the motor.
 24. A DC brush motor accordingto claim 23, wherein output power of said DC brush motor is in a rangefrom 1 to 300 W.
 25. A DC brush motor with a superior motor performanceindex (torque constant/motor volume) comprising: a permanent magnet,arranged on an inner perimeter of a housing, the permanent magnet beingformed as a thin-walled hollow cylinder shaped anisotropic rare earthbonded magnet having a maximum energy product greater than 25 MGOe andmagnetized with at least 4 magnetic poles; and an electromagnetic rotorcore arranged in the center of the motor.
 26. A DC brush motor accordingto claim 25, wherein output power of said DC brush motor is in a rangefrom 1 to 300 W.